Database Engineering Mastery

Complete database design, optimization, migration, and operations system. From schema design to production monitoring — covers PostgreSQL, MySQL, SQLite, and general SQL patterns.

Phase 1 — Schema Design

Design Brief

Before writing any DDL, fill this out:

project: ""
domain: ""
primary_use_case: "OLTP | OLAP | mixed"
expected_scale:
  rows_year_1: ""
  rows_year_3: ""
  concurrent_users: ""
  read_write_ratio: "80:20 | 50:50 | 20:80"
compliance: [] # GDPR, HIPAA, PCI-DSS, SOX
multi_tenancy: "none | schema-per-tenant | row-level | database-per-tenant"

Normalization Decision Framework

Denormalization triggers:

• Query joins > 4 tables consistently
• Read latency > 100ms on indexed queries
• Cache invalidation complexity exceeds denormalization maintenance
• Reporting queries block OLTP workloads

Naming Conventions

Tables:      snake_case, plural (users, order_items, payment_methods)
Columns:     snake_case, singular (first_name, created_at, is_active)
PKs:         id (bigint/uuid) or {table_singular}_id
FKs:         {referenced_table_singular}_id
Indexes:     idx_{table}_{columns}
Constraints: chk_{table}_{rule}, uq_{table}_{columns}, fk_{table}_{ref}
Enums:       Use VARCHAR + CHECK, not DB enums (easier to migrate)
Booleans:    is_, has_, can_ prefix (is_active, has_subscription)
Timestamps:  _at suffix (created_at, updated_at, deleted_at)

Column Type Decision Tree

Text < 255 chars, fixed set?     → VARCHAR(N) + CHECK
Text < 255 chars, variable?      → VARCHAR(255)
Text > 255 chars?                → TEXT
Whole numbers < 2B?              → INTEGER
Whole numbers > 2B?              → BIGINT
Money/financial?                 → NUMERIC(precision, scale) — NEVER float
True/false?                      → BOOLEAN
Date only?                       → DATE
Date + time?                     → TIMESTAMPTZ (always with timezone)
Unique identifier?               → UUID (distributed) or BIGSERIAL (single DB)
JSON/flexible schema?            → JSONB (Postgres) or JSON (MySQL)
Binary/file?                     → Store in object storage, reference by URL
IP address?                      → INET (Postgres) or VARCHAR(45)
Geospatial?                      → PostGIS geometry/geography types

Essential Table Template

CREATE TABLE {table_name} (
    id          BIGSERIAL PRIMARY KEY,
    -- domain columns here --
    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    created_by  BIGINT REFERENCES users(id),
    version     INTEGER NOT NULL DEFAULT 1,  -- optimistic locking
    
    -- soft delete (optional)
    deleted_at  TIMESTAMPTZ,
    
    -- multi-tenant (optional)  
    tenant_id   BIGINT NOT NULL REFERENCES tenants(id)
);

-- Updated_at trigger (PostgreSQL)
CREATE OR REPLACE FUNCTION update_modified_column()
RETURNS TRIGGER AS $$
BEGIN
    NEW.updated_at = NOW();
    NEW.version = OLD.version + 1;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER trg_{table_name}_updated
    BEFORE UPDATE ON {table_name}
    FOR EACH ROW
    EXECUTE FUNCTION update_modified_column();

Relationship Patterns

One-to-Many:

-- Parent
CREATE TABLE departments (id BIGSERIAL PRIMARY KEY, name VARCHAR(100) NOT NULL);
-- Child  
CREATE TABLE employees (
    id BIGSERIAL PRIMARY KEY,
    department_id BIGINT NOT NULL REFERENCES departments(id) ON DELETE RESTRICT,
    -- ON DELETE options: RESTRICT (safe default), CASCADE (children die), SET NULL
);
CREATE INDEX idx_employees_department_id ON employees(department_id);

Many-to-Many:

CREATE TABLE user_roles (
    user_id BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    role_id BIGINT NOT NULL REFERENCES roles(id) ON DELETE CASCADE,
    granted_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    granted_by BIGINT REFERENCES users(id),
    PRIMARY KEY (user_id, role_id)
);

Self-Referencing (hierarchy):

CREATE TABLE categories (
    id BIGSERIAL PRIMARY KEY,
    parent_id BIGINT REFERENCES categories(id) ON DELETE CASCADE,
    name VARCHAR(100) NOT NULL,
    depth INTEGER NOT NULL DEFAULT 0,
    path TEXT NOT NULL DEFAULT ''  -- materialized path: '/1/5/12/'
);
CREATE INDEX idx_categories_parent ON categories(parent_id);
CREATE INDEX idx_categories_path ON categories(path text_pattern_ops);

Polymorphic (avoid if possible, use if you must):

-- Preferred: separate FKs
CREATE TABLE comments (
    id BIGSERIAL PRIMARY KEY,
    post_id BIGINT REFERENCES posts(id),
    ticket_id BIGINT REFERENCES tickets(id),
    body TEXT NOT NULL,
    CONSTRAINT chk_one_parent CHECK (
        (post_id IS NOT NULL)::int + (ticket_id IS NOT NULL)::int = 1
    )
);

Phase 2 — Indexing Strategy

Index Type Selection

CREATE INDEX idx_users_email ON users(email)

CREATE INDEX idx_sessions_token ON sessions USING hash(token)

CREATE INDEX idx_products_tags ON products USING gin(tags)

CREATE INDEX idx_locations_geom ON locations USING gist(geom)

CREATE INDEX idx_events_created ON events USING brin(created_at)

CREATE INDEX idx_orders_pending ON orders(created_at) WHERE status = 'pending'

CREATE INDEX idx_orders_user ON orders(user_id) INCLUDE (status, total)

Indexing Rules

1. Always index: Foreign keys, columns in WHERE/JOIN/ORDER BY
2. Never index: Low-cardinality columns alone (boolean, status with 3 values) — combine in composite
3. Composite order: Most selective column first, then left-to-right matches query patterns
4. Watch write overhead: Each index slows INSERT/UPDATE. >8 indexes on a write-heavy table = review
5. Unused index audit: Run monthly — drop indexes with 0 scans

Find Unused Indexes (PostgreSQL)

SELECT schemaname, tablename, indexname, idx_scan, 
       pg_size_pretty(pg_relation_size(indexrelid)) as size
FROM pg_stat_user_indexes
WHERE idx_scan = 0 AND indexrelid NOT IN (
    SELECT conindid FROM pg_constraint WHERE contype IN ('p', 'u')
)
ORDER BY pg_relation_size(indexrelid) DESC;

Find Missing Indexes (PostgreSQL)

SELECT relname, seq_scan, seq_tup_read, 
       idx_scan, seq_tup_read / GREATEST(seq_scan, 1) as avg_tuples_per_scan
FROM pg_stat_user_tables
WHERE seq_scan > 100 AND seq_tup_read > 10000
ORDER BY seq_tup_read DESC;
-- High seq_scan + high seq_tup_read = missing index candidate

Phase 3 — Query Optimization

EXPLAIN Interpretation

EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT) SELECT ...;

Red flags in query plans:

Query Anti-Patterns & Fixes

1. SELECT * in production:

-- Bad: fetches all columns, breaks covering indexes
SELECT * FROM orders WHERE user_id = 123;
-- Good: explicit columns
SELECT id, status, total, created_at FROM orders WHERE user_id = 123;

2. N+1 queries:

-- Bad: 1 query for users + N queries for orders
SELECT id FROM users WHERE active = true;  -- returns 100 rows
SELECT * FROM orders WHERE user_id = ?;     -- called 100 times

-- Good: single JOIN or IN
SELECT u.id, o.id, o.total 
FROM users u
JOIN orders o ON o.user_id = u.id
WHERE u.active = true;

3. Functions on indexed columns:

-- Bad: can't use index on created_at
WHERE EXTRACT(YEAR FROM created_at) = 2025
-- Good: range scan uses index
WHERE created_at >= '2025-01-01' AND created_at < '2026-01-01'

-- Bad: can't use index on email  
WHERE LOWER(email) = 'user@example.com'
-- Good: expression index
CREATE INDEX idx_users_email_lower ON users(LOWER(email));

4. OR conditions killing indexes:

-- Bad: often causes Seq Scan
WHERE status = 'pending' OR status = 'processing'
-- Good: IN uses index
WHERE status IN ('pending', 'processing')

5. Pagination with OFFSET:

-- Bad: OFFSET 10000 scans and discards 10000 rows
SELECT * FROM products ORDER BY id LIMIT 20 OFFSET 10000;
-- Good: keyset pagination
SELECT * FROM products WHERE id > :last_seen_id ORDER BY id LIMIT 20;

6. COUNT(*) on large tables:

-- Bad: full table scan
SELECT COUNT(*) FROM events;
-- Good: approximate count (PostgreSQL)
SELECT reltuples::bigint FROM pg_class WHERE relname = 'events';
-- Or maintain a counter cache table

Window Functions Reference

-- Running total
SELECT id, amount, SUM(amount) OVER (ORDER BY created_at) as running_total FROM payments;

-- Rank within group
SELECT *, RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) as dept_rank FROM employees;

-- Previous/next row
SELECT *, LAG(amount) OVER (ORDER BY created_at) as prev_amount,
          LEAD(amount) OVER (ORDER BY created_at) as next_amount FROM payments;

-- Moving average
SELECT *, AVG(amount) OVER (ORDER BY created_at ROWS BETWEEN 6 PRECEDING AND CURRENT ROW) as ma_7 FROM daily_sales;

-- Percent of total
SELECT *, amount / SUM(amount) OVER () * 100 as pct_of_total FROM line_items WHERE order_id = 1;

CTE Patterns

-- Recursive: org chart traversal
WITH RECURSIVE org AS (
    SELECT id, name, manager_id, 1 as depth FROM employees WHERE manager_id IS NULL
    UNION ALL
    SELECT e.id, e.name, e.manager_id, o.depth + 1
    FROM employees e JOIN org o ON e.manager_id = o.id
    WHERE o.depth < 10  -- safety limit
)
SELECT * FROM org ORDER BY depth, name;

-- Data pipeline: clean → transform → aggregate
WITH cleaned AS (
    SELECT *, TRIM(LOWER(email)) as clean_email FROM raw_signups WHERE email IS NOT NULL
),
deduped AS (
    SELECT DISTINCT ON (clean_email) * FROM cleaned ORDER BY clean_email, created_at DESC
)
SELECT DATE_TRUNC('week', created_at) as week, COUNT(*) FROM deduped GROUP BY 1 ORDER BY 1;

Phase 4 — Migrations

Migration Safety Rules

1. Never rename columns/tables in production without a multi-step process
2. Never add NOT NULL without a DEFAULT on existing tables with data
3. Never drop columns that application code still references
4. Always test migrations on a copy of production data first
5. Always have a rollback plan (down migration)
6. Always take a backup before schema changes in production

Safe Migration Patterns

Add column (safe):

-- Step 1: Add nullable column
ALTER TABLE users ADD COLUMN phone VARCHAR(20);
-- Step 2: Backfill (in batches!)
UPDATE users SET phone = '' WHERE phone IS NULL AND id BETWEEN 1 AND 10000;
-- Step 3: Add NOT NULL after backfill
ALTER TABLE users ALTER COLUMN phone SET NOT NULL;
ALTER TABLE users ALTER COLUMN phone SET DEFAULT '';

Rename column (safe multi-step):

-- Step 1: Add new column
ALTER TABLE users ADD COLUMN full_name VARCHAR(200);
-- Step 2: Dual-write in application code (write to both old + new)
-- Step 3: Backfill
UPDATE users SET full_name = name WHERE full_name IS NULL;
-- Step 4: Switch application to read from new column
-- Step 5: Drop old column (after confirming no reads)
ALTER TABLE users DROP COLUMN name;

Add index without locking (PostgreSQL):

CREATE INDEX CONCURRENTLY idx_orders_customer ON orders(customer_id);
-- Takes longer but doesn't lock the table

Large table backfill (batched):

-- Don't: UPDATE millions of rows in one transaction
-- Do: batch it
DO $$
DECLARE
    batch_size INT := 5000;
    affected INT;
BEGIN
    LOOP
        UPDATE users SET normalized_email = LOWER(email)
        WHERE normalized_email IS NULL AND id IN (
            SELECT id FROM users WHERE normalized_email IS NULL LIMIT batch_size
        );
        GET DIAGNOSTICS affected = ROW_COUNT;
        RAISE NOTICE 'Updated % rows', affected;
        EXIT WHEN affected = 0;
        COMMIT;
    END LOOP;
END $$;

Migration File Template

-- Migration: YYYYMMDDHHMMSS_description.sql
-- Author: [name]
-- Ticket: [JIRA/Linear ID]
-- Risk: low|medium|high
-- Rollback: see DOWN section
-- Estimated time: [for production data volume]
-- Requires: [prerequisite migrations]

-- ========== UP ==========
BEGIN;

-- [DDL/DML here]

COMMIT;

-- ========== DOWN ==========
-- BEGIN;
-- [Rollback DDL/DML here]
-- COMMIT;

-- ========== VERIFY ==========
-- [Queries to confirm migration succeeded]
-- SELECT COUNT(*) FROM ... WHERE ...;

Phase 5 — Performance Monitoring

Key Metrics Dashboard

health_metrics:
  connections:
    active: "SELECT count(*) FROM pg_stat_activity WHERE state = 'active'"
    idle: "SELECT count(*) FROM pg_stat_activity WHERE state = 'idle'"
    max: "SHOW max_connections"
    threshold: "active > 80% of max = ALERT"
    
  cache_hit_ratio:
    query: |
      SELECT ROUND(100.0 * sum(heap_blks_hit) / 
             NULLIF(sum(heap_blks_hit) + sum(heap_blks_read), 0), 2) as ratio
      FROM pg_statio_user_tables
    healthy: "> 99%"
    warning: "< 95%"
    critical: "< 90%"
    
  index_hit_ratio:
    query: |
      SELECT ROUND(100.0 * sum(idx_blks_hit) / 
             NULLIF(sum(idx_blks_hit) + sum(idx_blks_read), 0), 2) as ratio
      FROM pg_statio_user_indexes
    healthy: "> 99%"
    
  table_bloat:
    query: |
      SELECT relname, n_dead_tup, n_live_tup,
             ROUND(100.0 * n_dead_tup / NULLIF(n_live_tup, 0), 2) as dead_pct
      FROM pg_stat_user_tables WHERE n_dead_tup > 10000
      ORDER BY n_dead_tup DESC LIMIT 10
    action: "VACUUM ANALYZE {table} when dead_pct > 20%"
    
  slow_queries:
    query: |
      SELECT query, calls, mean_exec_time, total_exec_time
      FROM pg_stat_statements
      ORDER BY mean_exec_time DESC LIMIT 20
    action: "Optimize top 5 by total_exec_time first"
    
  replication_lag:
    query: |
      SELECT EXTRACT(EPOCH FROM replay_lag) as lag_seconds
      FROM pg_stat_replication
    warning: "> 5 seconds"
    critical: "> 30 seconds"

Table Size Analysis

SELECT 
    relname as table,
    pg_size_pretty(pg_total_relation_size(relid)) as total_size,
    pg_size_pretty(pg_relation_size(relid)) as table_size,
    pg_size_pretty(pg_total_relation_size(relid) - pg_relation_size(relid)) as index_size,
    n_live_tup as row_count
FROM pg_stat_user_tables
ORDER BY pg_total_relation_size(relid) DESC
LIMIT 20;

Lock Monitoring

-- Find blocking queries
SELECT 
    blocked.pid as blocked_pid,
    blocked.query as blocked_query,
    blocking.pid as blocking_pid,
    blocking.query as blocking_query,
    NOW() - blocked.query_start as blocked_duration
FROM pg_stat_activity blocked
JOIN pg_locks bl ON bl.pid = blocked.pid
JOIN pg_locks kl ON kl.locktype = bl.locktype AND kl.relation = bl.relation AND kl.pid != bl.pid
JOIN pg_stat_activity blocking ON blocking.pid = kl.pid
WHERE NOT bl.granted;

Phase 6 — Backup & Recovery

Backup Strategy Decision

Backup Commands

# Logical backup (compressed)
pg_dump -Fc -Z 9 -j 4 -d mydb -f backup_$(date +%Y%m%d_%H%M%S).dump

# Restore
pg_restore -d mydb -j 4 --clean --if-exists backup_20260216.dump

# Schema only
pg_dump -s -d mydb -f schema.sql

# Single table
pg_dump -t orders -d mydb -f orders_backup.dump

# Physical backup
pg_basebackup -D /backup/base -Ft -z -P -X stream

Backup Verification Checklist

• Backup completes without errors
• Backup file size is within expected range (not suspiciously small)
• Restore to a test database succeeds
• Row counts match production (spot check 5 tables)
• Application can connect and query the restored database
• Run automated test suite against restored backup
• Backup encryption verified (if required)
• Offsite copy confirmed

Phase 7 — Security

Access Control Checklist

-- Create application role (least privilege)
CREATE ROLE app_user LOGIN PASSWORD 'use-vault-not-plaintext';
GRANT CONNECT ON DATABASE mydb TO app_user;
GRANT USAGE ON SCHEMA public TO app_user;
GRANT SELECT, INSERT, UPDATE, DELETE ON ALL TABLES IN SCHEMA public TO app_user;
-- NO: GRANT ALL, superuser, CREATE, DROP

-- Read-only role for analytics
CREATE ROLE analyst LOGIN PASSWORD 'use-vault';
GRANT CONNECT ON DATABASE mydb TO analyst;
GRANT USAGE ON SCHEMA public TO analyst;
GRANT SELECT ON ALL TABLES IN SCHEMA public TO analyst;

-- Row-Level Security (multi-tenant)
ALTER TABLE orders ENABLE ROW LEVEL SECURITY;
CREATE POLICY tenant_isolation ON orders
    USING (tenant_id = current_setting('app.tenant_id')::bigint);

SQL Injection Prevention

RULE 1: NEVER concatenate user input into SQL strings
RULE 2: Always use parameterized queries / prepared statements
RULE 3: Validate and whitelist table/column names if dynamic
RULE 4: Use ORMs for CRUD, raw SQL only for complex queries
RULE 5: Audit logs for unusual query patterns (UNION, DROP, --)

Data Protection

-- Encrypt sensitive columns (application-level)
-- Store: pgp_sym_encrypt(data, key) 
-- Read: pgp_sym_decrypt(encrypted_col, key)

-- Audit trail table
CREATE TABLE audit_log (
    id BIGSERIAL PRIMARY KEY,
    table_name VARCHAR(100) NOT NULL,
    record_id BIGINT NOT NULL,
    action VARCHAR(10) NOT NULL, -- INSERT, UPDATE, DELETE
    old_data JSONB,
    new_data JSONB,
    changed_by BIGINT REFERENCES users(id),
    changed_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    ip_address INET
);

-- Generic audit trigger
CREATE OR REPLACE FUNCTION audit_trigger() RETURNS TRIGGER AS $$
BEGIN
    INSERT INTO audit_log (table_name, record_id, action, old_data, new_data, changed_by)
    VALUES (
        TG_TABLE_NAME,
        COALESCE(NEW.id, OLD.id),
        TG_OP,
        CASE WHEN TG_OP != 'INSERT' THEN to_jsonb(OLD) END,
        CASE WHEN TG_OP != 'DELETE' THEN to_jsonb(NEW) END,
        current_setting('app.user_id', true)::bigint
    );
    RETURN COALESCE(NEW, OLD);
END;
$$ LANGUAGE plpgsql;

Phase 8 — PostgreSQL Configuration Tuning

Essential Settings by Server Size

Connection Pooling (PgBouncer)

[databases]
mydb = host=127.0.0.1 port=5432 dbname=mydb

[pgbouncer]
pool_mode = transaction          # transaction pooling (best for most apps)
max_client_conn = 1000           # accept up to 1000 app connections
default_pool_size = 25           # 25 actual DB connections per database
reserve_pool_size = 5            # extra connections for burst
reserve_pool_timeout = 3         # seconds before using reserve
server_idle_timeout = 300        # close idle server connections after 5 min

Phase 9 — Common Patterns

Soft Delete

-- Add to table
ALTER TABLE users ADD COLUMN deleted_at TIMESTAMPTZ;
CREATE INDEX idx_users_active ON users(id) WHERE deleted_at IS NULL;

-- Application queries always filter
SELECT * FROM users WHERE deleted_at IS NULL AND ...;

-- Or use a view
CREATE VIEW active_users AS SELECT * FROM users WHERE deleted_at IS NULL;

Optimistic Locking

UPDATE products SET 
    price = 29.99, 
    version = version + 1, 
    updated_at = NOW()
WHERE id = 123 AND version = 5;  -- expected version
-- If 0 rows affected → concurrent modification → retry or error

Event Sourcing Table

CREATE TABLE events (
    id BIGSERIAL PRIMARY KEY,
    aggregate_type VARCHAR(50) NOT NULL,
    aggregate_id UUID NOT NULL,
    event_type VARCHAR(100) NOT NULL,
    event_data JSONB NOT NULL,
    metadata JSONB DEFAULT '{}',
    version INTEGER NOT NULL,
    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    UNIQUE (aggregate_id, version)
);
CREATE INDEX idx_events_aggregate ON events(aggregate_id, version);
CREATE INDEX idx_events_type ON events(event_type, created_at);

Time-Series Optimization

-- Partitioned by month
CREATE TABLE metrics (
    id BIGSERIAL,
    sensor_id INTEGER NOT NULL,
    value NUMERIC(12,4) NOT NULL,
    recorded_at TIMESTAMPTZ NOT NULL
) PARTITION BY RANGE (recorded_at);

CREATE TABLE metrics_2026_01 PARTITION OF metrics
    FOR VALUES FROM ('2026-01-01') TO ('2026-02-01');
CREATE TABLE metrics_2026_02 PARTITION OF metrics
    FOR VALUES FROM ('2026-02-01') TO ('2026-03-01');

-- Auto-create future partitions via cron or pg_partman
-- Use BRIN index for time-series
CREATE INDEX idx_metrics_time ON metrics USING brin(recorded_at);

Full-Text Search (PostgreSQL)

-- Add search column
ALTER TABLE articles ADD COLUMN search_vector tsvector;
CREATE INDEX idx_articles_search ON articles USING gin(search_vector);

-- Populate
UPDATE articles SET search_vector = 
    setweight(to_tsvector('english', COALESCE(title, '')), 'A') ||
    setweight(to_tsvector('english', COALESCE(body, '')), 'B');

-- Search with ranking
SELECT id, title, ts_rank(search_vector, query) as rank
FROM articles, plainto_tsquery('english', 'database optimization') query
WHERE search_vector @@ query
ORDER BY rank DESC LIMIT 20;

JSONB Patterns

-- Store flexible attributes
CREATE TABLE products (
    id BIGSERIAL PRIMARY KEY,
    name VARCHAR(200) NOT NULL,
    attributes JSONB NOT NULL DEFAULT '{}',
    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
);

-- Index specific JSON paths
CREATE INDEX idx_products_color ON products((attributes->>'color'));
-- Or GIN for any key lookups
CREATE INDEX idx_products_attrs ON products USING gin(attributes);

-- Query patterns
SELECT * FROM products WHERE attributes->>'color' = 'red';
SELECT * FROM products WHERE attributes @> '{"size": "large"}';
SELECT * FROM products WHERE attributes ? 'warranty';

Phase 10 — Operational Runbooks

Emergency: Database Overloaded

-- 1. Find and kill long-running queries
SELECT pid, NOW() - query_start as duration, query 
FROM pg_stat_activity WHERE state = 'active' AND query_start < NOW() - INTERVAL '5 minutes'
ORDER BY duration DESC;

-- Kill a specific query
SELECT pg_cancel_backend(pid);    -- graceful
SELECT pg_terminate_backend(pid); -- force

-- 2. Check for lock contention (see Phase 5)

-- 3. Reduce max connections temporarily
-- In pgbouncer: pause database, reduce pool, resume

-- 4. Check if VACUUM is needed
SELECT relname, n_dead_tup, last_autovacuum FROM pg_stat_user_tables 
WHERE n_dead_tup > 100000 ORDER BY n_dead_tup DESC;

Emergency: Disk Full

# 1. Check what's consuming space
du -sh /var/lib/postgresql/*/main/ 2>/dev/null || du -sh /var/lib/mysql/

# 2. Clean up WAL files (PostgreSQL) — CAREFUL
# Check replication slot status first
SELECT slot_name, active FROM pg_replication_slots;
# Drop inactive slots consuming WAL
SELECT pg_drop_replication_slot('unused_slot');

# 3. VACUUM FULL largest tables (locks table!)
VACUUM FULL large_table;

# 4. Remove old backups / logs
find /backups -name "*.dump" -mtime +7 -delete

Weekly Maintenance Checklist

• Review slow query log (top 10 by total time)
• Check index usage stats — drop unused, add missing
• Verify backup success and test restore
• Check table bloat — schedule VACUUM where needed
• Review connection count trends
• Check disk space trajectory
• Review replication lag
• Update table statistics:

  ANALYZE;

Phase 11 — Database Comparison Quick Reference

Quality Scoring Rubric (0-100)

Score interpretation: <40 = Critical risk | 40-60 = Needs work | 60-80 = Solid | 80-90 = Professional | 90+ = Expert

Natural Language Commands

• "Design a schema for [domain]" → Phase 1 full design process
• "Optimize this query: [SQL]" → EXPLAIN analysis + rewrite
• "Add an index for [query pattern]" → Index type selection + creation
• "Write a migration to [change]" → Safe migration with rollback
• "Audit this database" → Full scoring across all dimensions
• "Set up monitoring for [database]" → Phase 5 dashboard queries
• "Review this schema" → Naming, types, constraints, relationships check
• "Help me with [PostgreSQL/MySQL/SQLite] [topic]" → Platform-specific guidance
• "Troubleshoot slow queries" → pg_stat_statements analysis + top fixes
• "Plan a backup strategy" → Phase 6 decision framework
• "Make this table multi-tenant" → RLS + tenant_id pattern
• "Convert this to use partitioning" → Phase 9 time-series pattern